The objective of these studies is to measure interstitial fluid pressure (Pf) in lungs and to relate these to factors affecting fluid filtration and edema formation. Two approaches are being taken to direct measurement of Pf. At the macroscopic level, one mm diameter fluid filled catheters are inserted into periolbar/perivascular spaces from the hilum of excised lungs. Initial data show that Pf is more negative than has been presumed, ranging as much as 30-60 cm H2O less than pleural pressure. At the microscopic level, glass micropipets with tip diameters in the order of 0.1 micron and filled with hypertonic saline solution are attached to a servo-null micropressure system. Pipets are driven through the pleurae of inflated perfused lungs into various fluid compartments (capillary lumen, pericapillary interstitial space, alveolar subphase) to obtain local Pf.